Is Magnesium Oxide used as a refractory agent?

Yes, it is.

Magnesium oxide is used as a refractory agent in the construction of crucibles. A crucible is a container which is intended to be placed in extremely high temperatures for the sake of heating its contents. Since the crucible itself must not be affected by the heat, it must be made with substances which retain their properties at high heat. Magnesium oxide is such a substance, so it is used in this manner.